Realization of a two-dimensional Weyl semimetal and topological Fermi strings

A two-dimensional (2D) Weyl semimetal, akin to a spinful variant of graphene, represents a topological matter characterized by Weyl fermion-like quasiparticles in low dimensions. The spinful linear band structure in two dimensions gives rise to distinctive topological properties, accompanied by the emergence of Fermi string edge states. We report the experimental realization of a 2D Weyl semimetal, bismuthene monolayer grown on SnS(Se) substrates. Using spin and angle-resolved photoemission and scanning tunneling spectroscopies, we directly observe spin-polarized Weyl cones, Weyl nodes, and Fermi strings, providing consistent evidence of their inherent topological characteristics. Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials.


REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): I have read the reply of the authors and followed the changes.I thank the authors for their good work.I suggest to publish as is.
Reviewer #2 (Remarks to the Author): In their resubmission, the authors have rewritten the manuscript considerably in an effort to address the criticism raised by myself and the other referees.I appreciate that effort, and feel that the manuscript has improved quite a bit.Some erroneous or misleading statements were removed, including claims of applicability, which I still find not to substantiated by the present data.Now, the manuscript presents careful experiments on a specific material that realizes a 2D Weyl semimetal.The shown data supports the claims, and compares well with the theory.
As the authors acknowledge, their 2D Weyl semimetal is not really a phase, but a fine-tuned situation.Despite that, the authors still talk of their 2D Weyl semimetal as being a "phase".This is wrong: it's a critical situation at the transition between two extended phases.This point is not emphasised enough in the current manuscript.For example the figure showing the evolution "Gapped 2D Dirac -> Rashba Splitting -> Gapless 2D Weyl", is not represented in the main text.Similarly, statements made in the replies, e.g."It is technically challenging to realize the 2D Weyl semimetal for it is an "accidental" critical phase that is proximal to trivial insulators and quantum spin Hall insulators.To reach this critical phase, one needs to tune a coupling parameter to the critical value."are not fully represented in the main text.
Instead, the main text now states that 2D Weyl semimetals are a new topological state.They underpin this with the notion of a winding number.This indeed is technically not wrong, but the winding number is only "quantized" to +/-pi if the gap vanishes.There is no topological protection against small perturbations.Given all of the above, I find that the framing of the results as showing a novel topological phase is simply overselling.It's not a phase, nor is it topologically protected in a way similar to the neighbouring true phases.Instead, the authors perform very nice and exhaustive experiments, and their data is constant with realising a fine-tuned state at the transition between two gapped topological phases.Given this assessment, I do not support publication of the manuscript in Nature Communications in the present form.
Reviewer #3 (Remarks to the Author): The authors have carefully considered my concerns and made substantial effort to answer my questions in a proper way.Some additional STM and ARPES experiments were performed and results were included in the manuscript that certainly increased its quality.I recommend the manuscript for publication.

Referee 2:
In their resubmission, the authors have rewritten the manuscript considerably in an effort to address the criticism raised by myself and the other referees.I appreciate that effort, and feel that the manuscript has improved quite a bit.Some erroneous or misleading statements were removed, including claims of applicability, which I still find not to substantiated by the present data.Now, the manuscript presents careful experiments on a specific material that realizes a 2D Weyl semimetal.The shown data supports the claims, and compares well with the theory.
As the authors acknowledge, their 2D Weyl semimetal is not really a phase, but a fine-tuned situation.Despite that, the authors still talk of their 2D Weyl semimetal as being a "phase".This is wrong: it's a critical situation at the transition between two extended phases.This point is not emphasised enough in the current manuscript.For example the figure showing the evolution "Gapped 2D Dirac -> Rashba Splitting -> Gapless 2D Weyl", is not represented in the main text.Similarly, statements made in the replies, e.g."It is technically challenging to realize the 2D Weyl semimetal for it is an "accidental" critical phase that is proximal to trivial insulators and quantum spin Hall insulators.To reach this critical phase, one needs to tune a coupling parameter to the critical value."are not fully represented in the main text.
Instead, the main text now states that 2D Weyl semimetals are a new topological state.They underpin this with the notion of a winding number.This indeed is technically not wrong, but the winding number is only "quantized" to +/-pi if the gap vanishes.There is no topological protection against small perturbations.Given all of the above, I find that the framing of the results as showing a novel topological phase is simply overselling.It's not a phase, nor is it topologically protected in a way similar to the neighbouring true phases.Instead, the authors perform very nice and exhaustive experiments, and their data is constant with realising a fine-tuned state at the transition between two gapped topological phases.Given this assessment, I do not support publication of the manuscript in Nature Communications in the present form.